In a digital color copier, various components perform a series of processes in order to reproduce an original image as shown in FIG. 1. In general, the processes initially involve scanning an original image 1 in red, green and blue (RGB) values in an analog format via an optical scanner unit 2. An analog-to-digital (A/D) converter 3 converts the scanned analog signal into digital signal. Since each scanner 2 has its own input characteristics, a scanner correction unit 4 corrects the digits data according to the input characteristics. For the output purpose, the corrected digital signal is converted into cyan, magenta, yellow, and black (CMYK) signals by a color conversion unit 5. An image processing unit 6 further processes the CMYK signals. For example, the image processing involves enlargement, reduction and cutting a certain portion of the image. Prior to outputting an image, an intensity gamma correction unit corrects the intensity level according to the output characteristics of an output unit 9. Similarly, a chromaticity gamma correction unit 8 also corrects the chromaticity values according to the output characteristics of the output unit 9 prior to outputting an image.
In the above described digital reproduction processes, an amount of color information is an important factor in determining the speed of each process. For example, assuming that each pixel is represented by 32-bit data for the CMYK value (8-bit for each color component C, M, Y or K), at 400 dpi, the total amount of data representing an image on A-4 size paper is approximately 62 mega bytes (MB). In order to reproduce ten pages of A-4 color image per minute (10 ppm) 10.3 MB of information must be processed each second. In other words, referring back to FIG. 1, the units such as the image processing unit 6 must be able to process information faster than at least 10.3 MB/sec. In general, because of the above required high performance, certain crucial processing components are costly and cannot be replaced with less expensive slow units.
One approach to use the slow processing components is to reduce an amount of data by conventional data compression techniques. In general, the conventional data compression techniques are used to reduce the amount of data poor to storing in a storage device so that the storage space is saved. In another area of applications, the conventional data compression techniques are useful to reduce the data size prior to transmitting the data in order to the save transmission line capacity. However, since the compressed data generally contains less than original information, the compressed data needs to be restored to the original uncompressed data via a decompression process before the data is image processed. In other words, the compressed data is not usually suitable for image processing such as enlargement and reduction. For this reason, the conventional compression techniques are not particularly useful for efficiently processing image data.
Another approach in reducing the data size is approximation of the original data. Since the ability of humans to differentiate the gradation values rapidly decreases as the resolution of the output image increases, at a high resolution, the gradation appears indistinguishable to human eyes. P. G. Loetling, visual Performance and Image Coding, SPIE/OSA, Image Processing 74 (1976). The input color data is approximated by a fixed reduction rate using an error diffusion method or a dithering method. For example, Japanese Patent applications 2-210963 and 6-98157 disclose improved error diffusion techniques. However, these approximation techniques are applied to approximate the input data only for the purpose of outputting the input image.
As described above, there has been a need for efficiently image processing a large amount of information such as color image data without relying upon expensive high-speed processing components without sacrificing the reproduction speed and reproduced image quality. In the alternative, there has been also a need for improving the processing speed without requiring a higher-speed image processing components.